The present invention relates to a data transmission apparatus utilized in a digital mobile radio communication equipment and the like.
FIG. 1 is a diagram showing a structure of a conventional data transmission apparatus. In FIG. 1, a reference numeral 1 represents a signal source for outputting transmission data such as voice data, 2 represents an encoder performing error coding on the above-mentioned transmission data, 3 represents a modulator for modulating the transmission data into signals for transmission, and 4 represents a transmitting antenna for radiating transmission signals. 5 represents a receiving antenna for receiving transmission signals, 6 represents a demodulator for demodulating received signals, 7 represents an amplitude limiter for limiting the amplitude of received signals and 8 represents a decoder for obtaining decoded data.
Next, the operation of the above-mentioned conventional example will be described. In FIG. 1, the digital data outputted from the signal source 1 are code-converted by error correction coding such as convolutional coding in the encoder 2, applied with modulation such as frequency-shift keying (FSK) and phase-shift keying (PSK) by the modulator 3, and transmitted from the transmitting antenna 4. The transmitted signals are received by the receiving antenna 5 and demodulated by the demodulator 6. Here, a value at an identification point is outputted as it is from the demodulator 6 as a demodulator output X. In the amplitude limiter 7, in order to perform decoding using Euclidean distance such as Viterbi decoding (which means correction of errors produced in a transmission line by releasing coding coded in the encoder 2) with soft decision by the decoder 8, a demodulator output X is limited within a range from 1 to -1 as shown in FIG. 2 so as to be inputted to the decoder 8 as a decoder input Y. The output of the decoder 8 is processed as decoded data.
In the above-mentioned conventional data transmission apparatus, it is possible to correct bit errors in digital signals by coding with the encoder and decoding with the decoder and decoding is performed in the decoder with soft decision. Thus, there is an advantage that error correction capacity is also higher as compared with a case of hard decision.
In the above-mentioned data transmission apparatus, however, it is only made to limit the demodulator output X as the decoder input Y by the amplitude limiter 7 between 1 and -1. Accordingly, the error rate characteristic is improved when soft decision is made by the decoder 8 as compared with a case of hard decision, but the effects of soft decision cannot be produced satisfactorily.